Apparatus for electrostatically charging insulating image surfaces for electrophotography



Dec. 18, 1956 L. E. WALKUP 2,774,921

APPARATUS FOR ELECTROSTATICALLY CHARGING INSULATING IMAGE SURFACES FOR EILECTROPHOTOGRAPHY Filed April 25, 1955 Po/enfia/ Source INVENTOR Lewis E. Wolkup ATTORNEYS.

APPARATUS FOR ELECTROSTATICALLY CHARG- ING INSULATING IMAGE SURFACESFOR ELEC- TROPI-IQTOGRAPHY Lewis E. Walkup, Columbus, Ohio, assignor, by mesne assignments, to The Haloid Company, Rochester, N. Y., a corporation of New York Application April 23, 1953, Serial No. 350,578

2 Claims. (Cl. 317-262) This invention relates to a method and apparatus for electrostatically charging surfaces used in electrophotography and is a continuation-in-part of my applications Serial No. 76,834, filed February 8, 1949 now abandoned, and Serial No. 161,286, filed May 11, 1950, now abandoned, each entitled Method and Apparatus for Electrostatically Charging Insulating Image Surfaces for Electrophotography. This invention relates to the electrophotographic and eleotroprinting processes wherein a surface bearing an image made up of electrical charges is dusted with a developer powder to form a powder image which may be transferred and affixed to a second surface, and has as an object the provision of a simple, effective, and practical method and apparatus for imposing an electrostatic charge onto such a surface by corona discharge.

Another object of this invention is to provide a novel method and apparatus for supplying a uniformly distributed electrostatic charge over the insulating surface of an electrophotographic plate.

Yet another object of this invention is to provide an electrostatic charging method and apparatuswherein operation at lower voltages than are required by prior corona discharge devices is possible.

Other objects and advantages of the present invention will be more readily apparent in view of the following detailed disclosure and descriptionthereof, especially when read in conjunction with the accompanying drawings wherein:

Fig. 1 is a top view of atypical embodiment of the present invention;

Fig. 2 is a side view of the same embodiment, and

Fig. 3 is a section on line 33 of Fig. 1 showing the manner in which a spring contact may be used to complete the electrical circuit of the illustrated embodiment.

The invention is intended for use in conjunction with electrically insulating surfaces, such as. a photoconductive insulating surface of a suitable material (selenium, sulfur or anthracene, for example) on a conductive backing plate employed in electrophotographic operations, or electrically insulating surfaces or images that may be attached to a printing drum and employed in continuous electroprinting processes for making many copies of an original at high speed. In the electrophotographic process, the insulating surface must be charged electrostatically before exposing, and in the continuous printing operation electrostatic charging is necessary priorto dusting with a developer powder and the present invention is directed toward an improved method and structure for imposing electrostatic charges on such insulating surfaces.

Referring now to the drawings wherein like reference numbers identify the same elements in the three views, a photoconductive insulating surface 1 is shown in the form of a coating on a conductive backingplate 7. A conducting bar or support 2 is shown which may be connected through conductor 3 andswitch 13 with a suitable potential supply 6. Permanently attached to the bar 2 is a pliable element 4 formed of thigh-resistance electrically nited States Patent "ice conductive material which extends downwardly toward the insulating surface 1, and in the form here illustrated is provided at its lower end with a fringed portion 5 affording a multiplicity of separate fibers extending downwardly in generally parallel relation and in close proximity to each other so as to provide a brush-like structure. The fibers of the fringed portion 5 are shown in yieldable contact with and arranged to wipe over the insulating surface 1 as the plate and bar 2 are moved relatively to each other to effect endwise movement of the wiping element in relation to the plate consisting of the insulating layer 1 and the conductive backing 7. Satisfactory results may also be obtained if the lower end of the pliable element 4 is not fringed but has its bottom edge flexing against and wiping over the surface 1. Furthermore, the lower edge of the pliable element 4 need not contact the surface 1, but satisfactory results may be obtained with this lower edge spaced somewhat above and out of contact with surface 1.

In the embodiment shown in the drawings, relative movement between the flexible element 4 and the photo conductive surface 1 is effected by having the plate consisting of insulating layer 1 and conductive backing 7 move under the pliable element 4. Mechanism permitting such movement may comprise a table or other support 12 provided with conveyor means comprising belts 101. These belts 1'010 may be driven by means of a drive belt 15 attached to shaft 19 and actuated by motor 14. Shafts 20-40 need not be driven but may merely rotate freely in bearings as illustrated. The electrostatic charging means may be supported by elements 1-717 which are fastened to table 12 in any convenient manner. In operation, the illustrated device functions as follows: Belts 1ii10 are activated by means of drive belt 15 running off the shaft of motor 14 to drive shaft 19; A plate comprising photoconductive insulating layer 1 and conductive backing 7 is placed on the moving belts Ill-40. Switch 13 is closed, thus energizing flexible element 4 through conducting bar 2 and electrical lead 3. As the plate passes under flexible element 4, fringed portion 5 contacts insulating surface 1, while the conductive backing 7 contacts spring clip 11 thereby completing the electrical circuit. An electric charge is sprayed from the lower edge or tips of fibers 5 onto the surface 1, providing a uniformly distributed electrostatic charge over surface 1. The plate is then ready to beexposed to light in case it comprises an electrophotograph'ic plate; or to be dusted with a developer powder in the case where insulating surface 1 constitutes a printing image forming part of a continuous printing machine.

In the interest of safety, a shield 8 of insulating material may be provided around flexible element 4, as shown in Figs. 1 and 2. A portion of Fig. 1 has been cut away to show contact element 11 and Fig. 3 illustrates more clearly how this element 11, which is connected to the potential source 6 through grounded lead 18, contacts the conductive backing 7 of the plate as it passes under the discharge mechanism.

It has been discovered that the pliable element 4 may consist of any suitable high-resistance electrically conductive material. The material must not be highly conductive else the element 4- would act to short-circuit the current whenever any portion of the element touches a hole or weak spot in the insulating surface layer l, or whenever any portion of the element touches an exposed edge portion of the backing plate 7. This is especially objectionable in the case of printing plates for electrical printing whenever a shorting out occurred at a hole or at a fiaw in the coating.

Just as it is required that the pliable element 4 be not too conductive electrically, it is also important that it be not too resistant to electric current flow, for if the resistivity of the element is too high, then it will conduct no electricity down to the insulating layer, or it will require excessively high potentials to do so.

The resistances of various materials which are satisfactory for use as the charging element in this invention have been determined by measuring the resistance between probes spaced 1 centimeter apart on a strip of the material 1 centimeter wide. Materials having resistances, so measured, in the range from about 10,000 ohms to about 100 megohms have been found to be satisfactory. This obviously eliminates the more conductive materials from consideration as charging elements. For example, the resistance of copper, silver, and other common metals measured in this manner is less than 1 ohm. Thus, metals and correspondingly high-conductivity materials are unsuitable for the charging element of this invention.

Examples of materials, which are suitable for the charging elements in this invention, include paper, cloth, certain vegetable fibers, glass cloth which has been rendered slightly conductive by met-alizing or coating with hygroscopic materials, such as glycerin or various salts, various fibrous materials, bristles, etc.

Specifically, the following examples list resistance values of materials found suitable for charging insulating layers in accordance with this invention, although such examples are not to be interpreted as restrictive or in any way limiting.

Example I A paper having a resistance of 20 megohins, measured as above described, was used in laying down a dense, uniform electrostatic charge on an electrophotographic plate. The voltage applied to the supporting bar was 1400 volts (peak) and the paper element was moved across the plate surface at a speed of 1% inches per second.

Example II A black cloth material having a resistance of about 400,000 ohms produced a uniform charge on an electro photographic plate surface at 1400 volts (peak) and a speed of 1% inches per second.

Example III A paper material having a resistance of about 10,000 ohms produced a uniform charge on the surface of an electrophotographic plate at a charging potential of only -700 volts (peak) and at a speed of 1% inches per second.

In each of the examples cited above, the presence of the electrostatic charge on the electrophotographic plate was determined by dusting the plate with an electroscopic powder which adhered to the charged areas. By way of contrast to the above examples, tests with a cloth material having a resistance of about 2000 megohms resulted in no charge being laid down on the electrophotographic plate even at potentials as high as 8500 volts (peak).

An outstanding advantage of the present invention is that it permits the use of charging voltages which are considerably lower than those required in prior devices, such as those employing metal needles in spaced relation to the insulating surface. In addition to providing an obviously increased safety factor, this characteristic also provides for an economic advantage which inherently attends the reduction in requirement for voltage output of the power supply. In this connection, it is noted that any suitable potential source may be used, either D. C. or rectified A. C. being satisfactory. Voltage values in the range 200-2000 volts are operable in the present device.

The speed of relative movement between plate and charging elements, mentioned in the examples above, is in conformity with present practice in electrophotography. If faster speeds are desired, a charging element having a resistance value of the lower magnitude should be selected. Speed could also be increased by providing a plurality of charging units each with supporting bar, voltage source, and pliable charging element.

While the invention has been described with reference to the particular structure herein shown, it is not limited to the details set forth. For example, a device wherein the charging mechanism moves over the surface of the plate would be equally satisfactory to provide relative movement, and this application is intended to cover such other methods or departures as may come within the purpose of the improvement or the scope of the following claims.

What is claimed is:

1. Apparatus for charging a xerographic plate having a photoconductive insulating layer, said apparatus comprising support means for said plate, a high resistance pliable continuous layer charging electrode of substantially uniform resistance of at least 10 ohms when measured between probes spaced one centimeter apart on a strip of material one centimeter wide positioned to engage the surface of said layer while the plate is positioned on said support means, a conductive contact spaced from and substantially parallel with said layer and secured in contact with the charging electrode across said layer, means for moving the electrode and plate with respect to each other and in a direction transverse to said electrode to bring the electrode progressively into contact with the working area of said layer, an electrical potential source to apply a potential in the range of about 200 to 2000 volts, and connectors to connect the terminals of the potential source to said conductive contact and to the plate when in position on said support means, respectively, whereby during the application of potential from said potential source charge is deposited and trapped on the surface of said layer by progressively withdrawing said charging electrode from the surface of said layer while the flow of electric charge through said charging electrode is restricted.

2. Apparatus for charging a Xerographic plate having a photoconductive insulating layer, said apparatus comprising support means for said plate, a high resistance pliable continuous layer fibrous charging electrode of substantially uniform resistance of at least 10 ohms when measured between probes spaced one centimeter apart on a strip of material one centimeter wide positioned to engage the surface of said layer while the plate is positioned on said support means, a conductive contact spaced from and substantially parallel with said layer and secured in contact with the charging electrode across said layer, means for moving the electrode and plate with respect to each other and in a direction transverse to said electrode to bring the electrode progressively into contact with the working area of said layer, an electrical potential source to apply a potential in the range of about 200 to 2000 volts, and connectors to connect the terminals of the potential source to said conductive contact and to the plate when in position on said support means, respectively, whereby during the application of potential from said potential source charge is trapped and deposited on the surface of said layer by progressively withdrawing said charging electrode from the surface of said layer while the flow of electric charge through said charging electrode is restricted.

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